The removal of hydrogen sulfide from hydrocarbon streams, especially gas streams, is often required in order to meet many pipeline and storage regulations. Hydrogen sulfide and mercaptans are toxic and have an offensive odor, and these components can be adverse to downstream process equipment. Even in situations where the gas stream is flared, the presence of hydrogen sulfide and mercaptans result in the generation of pollutants such as sulfur dioxide and removal of these sulfur compounds may be mandated.
Numerous processes have been proposed for the removal of hydrogen sulfide and mercaptans including adsorption, absorption and reactions with chemical agents. Since it is desired to remove hydrogen sulfide and mercaptans at the source of the hydrocarbon stream in order to permit pipeline transport and storage or flaring, the processes need to be able to be operated under the conditions of the environment at the source. Often, the sources are remotely located and may be subject to harsh environments such as would be the case with natural gas wells. Because of such remote locations, minimization of operating problems is particularly desirable.
One type of process for removal of hydrogen sulfide uses hydrogen sulfide scavengers that react with hydrogen sulfide to provide a substantially non-toxic compound or a compound which can be removed from the hydrocarbon. Currently most frequently used hydrogen sulfide scavengers are hexahydrotriazines from monoethanolamine (MEA triazine) and methylamine (MMA triazine). These triazines are readily deployable in scrubbers or in a production train that can be physically located adjacent to hydrocarbon sources such as gas wells, are effective scavengers, and result in the formation of dithiazines. Moreover MEA triazine and MMA triazine are readily available at reasonable costs. Nevertheless, especially under colder conditions, these triazines are susceptible to the formation of solids, especially dithiazine deposits, that can result in operational difficulties and the necessity for maintenance of the scrubber system. Removal of solid deposits can be difficult and result in lost operational time. Recently, Taylor in United States published patent application no. 2012/0247515 summarized the problems caused by amorphous solids formation stating that:                “Cleaning procedures are time consuming and difficult. Often, the equipment has to be taken off-line so that the deposits can be manually chipped away. The industry places much effort and incurs great cost in the treatment of amorphous dithiazine buildup.” (paragraph 0003)Taylor proposes the use of hydrogen peroxide to react and dissolve amorphous dithiazine deposits.        
Taylor, et al., “Identification of the Molecular Species Responsible for the Initiation of Amorphous Dithiazine in Laboratory studies of 1,3,5-Tris (hydroxyethyl)-hexahydro-s-triazine as a Hydrogen Sulfide Scavenger”, Industrial & Engineering Chemistry Research, 2012, 51, 11613-11617, note that MEA triazine can form cubic crystals or amorphous dithiazine solids. The authors relate at page 11613 that when the MEA triazine is spent to a high-level, or when the reaction with hydrogen sulfide has proceeded very far along, it's pathway to the dithiazine exceeds solubility in the aqueous medium and comes out of solution as a lower, highly dense liquid layer. They state that once this liquid layer is formed, solids can occur. While the crystalline dithiazine can be melted or removed with an appropriate solvent, the amorphous dithiazine is typically insoluble in all organic solvents and does not melt. The authors identified a potential mechanism for the formation of the amorphous dithiazine involving an attack by bisulfide on the nitrogen moiety of the dithiazine to form complex oligomeric or polymeric structures.
Taylor, et al., state in the last paragraph on page 11613:                “Avoidance is the best current strategy for amorphous dithiazine, either by preventing the initial phase separation with co-solvency or ensuring that the degree of spent does not reach a level sufficient to cause its formation.”Referencing their prior work, the authors state at page 11614 that the presence of a terminal hydroxyl functionality in the hexahydrotriazines molecule is essential for the amorphous dithiazine to form. If this is blocked with a methoxy group, as with 2-methoxy ethyl or 3-methoxy propyl hexahydrotriazines, no solidification of the separated dithiazine layer was observed by them at complete spent condition.        
MMA triazine tends to form crystals when spent although it is a very reactive hydrogen sulfide scavenger. Due to the common method of manufacture of the MMA triazine, it is usually present in an aqueous concentrate containing about 30 to 40 weight percent MMA triazine. The dithiazine from MMA triazine is subject to additional attack by these sulfur compounds to results in ring opening. MMA triazine tends to be more expensive than MEA triazine.
Accordingly, a need still exists for hydrogen sulfide scavengers that are cost-effective, can be employed at remote locations in existing scrubbing equipment, and do not result in the formation of crystalline or amorphous solids.
Although the commercial hydrogen sulfide scavenging products today are based on MEA triazine and MMA triazine, investigators have proposed numerous triazines for use as hydrogen sulfide scavengers. Au, et al., in U.S. Pat. No. 4,830,827 discloses a wide variety of hexahydrotriazine derivatives including acyclic, heterocyclic, hydroxy and mercaptans of substituted acyclic heterocyclic, amido, acyl, ether, thioether, mercapto and the like derivatives. The compounds are stated to be capable of inhibiting corrosion. Example 4 illustrates the corrosion protection afforded by the compounds in the presence of hydrogen sulfide at a concentration of 50 ppm. Rivers, et al., in U.S. Pat. Nos. 5,347,004 and 5,554,349 discloses the use of a mixture of amines wherein at least one of the amines is a hexahydrotriazines having an alkoxyalkylene radical such as a mixture of MMA triazine and 1,3,5 tris-(3-methoxypropyl)-hexatriazine (MOPA triazine). The patentees provide no disclosure regarding a solids problem. They state at column 6, lines 12 et seq., that it has been discovered that where both methylamine and MOPA are used together to make the main mixture, that the effectiveness of the mixture increases with increasing MOPA portion relative to methylamine. They further add that since MOPA is presently relatively more expensive than methylamine, economic considerations may prefer relatively more methylamine as the main component.
Taylor in United States published patent application no. 2011/0220551 discloses hydrogen sulfide scavengers that are triazines made from an aldehyde and at least one aminoalkylalkanolamine and at least one primary alkyl amine. The manufacturing process is said to produce a mixture of triazines. In comparative example 4, the applicant discloses two syntheses of a mixed triazine containing moieties of differing hydroxyethyl and methyl substituents. Example 5 reports that the activity of the mixed triazines of Example 4 is essentially the same performance as a mixed triazine of aminoethylethanolamine and t-butyl amine No information regarding solids formation is disclosed.
Salma, et al., in U.S. Pat. No. 6,663,841 disclose a process form scavenging hydrogen sulfide from supercritical and/or liquid carbon dioxide. At column 2, lines 60 to 63, the patentees state:                “In one non-limiting embodiment, the preferred hexahydrotriazines include, but are not necessarily limited to, 1,3,5-tri-(2-hydroxyethyl)-hexahydro-S-triazine; 1,3,5-trimethylhexahydro-1,3,5-triazine; or mixtures thereof.”No specific disclosure is made of any mixture composition.        
Sullivan, et al., in U.S. Pat. No. 5,744,024 propose the use of trisubstituted hexahydrotriazines in the presence of quaternary ammonium compound to treat sour gas and liquid hydrocarbon. The preferred scavenging triazines are substituted with alkoxyalkyl, alkyl, or alkanol moieties such as MOPA triazine, MMA triazine and MEA triazine. Salma, et al., in U.S. Pat. No. 7,438,887 disclose hexahydrotriazine scavengers having both hydroxyalkyl and alkylamine functionality. The patentees state that the triazines remove hydrogen sulfide rapidly and with high capacity and are manufactured from relatively low-cost materials. The patentees provide no information regarding solids formation.
Significant challenges are faced in developing new compositions for scavenging hydrogen sulfide. For instance, the scavenger should be capable of being used in relatively noncomplex equipment, such as existing scrubbers, at the site of the source of the hydrocarbon stream. Hence, the scavenger should be soluble in a delivery system such as water and be tolerant of other components that may be present in the hydrocarbon stream such as carbon dioxide. The scavenger needs to be economically competitive with existing scavengers including scavenging effectiveness and rate, maintenance and other operational costs and the cost of the scavenger itself. Accordingly, significant hurdles exist for the development of a scavenger, let alone a scavenger that also avoids formation of crystalline or amorphous solids.